IR Functional Groups
Why do we ignore everything to the right of 1500 cm⁻¹ in IR?
This is the "fingerprint region" - too complex to interpret reliably
What is the approximate chemical shift range for alkane CH₃ groups?
~0.9 ppm
How many peaks appear in a triplet?
3
What does UN# stand for and what is the formulate to calculate it?
Unsaturation number (or degree of unsaturation)
Un# = (2C-H-X+N+2)/2
What are the 3 steps for solving a spectroscopy problem?
1) Find UN#
2) Look for functional groups on IR
3) Look at H-NMR
An IR spectrum shows a broad absorption around 3000 cm⁻¹. What functional group is present?
O-H (alcohol or carboxylic acid)
A doublet appears at 9-10 ppm. What functional group is present?
R-CHO (aldehyde)
A quartet integrating for 2H is next to what?
A CH₃ group (3 neighboring protons)
Calculate UN# for C₆H₁₂
UN# = (2(6)-12+2)/2 = 1
A molecule has UN# = 4 and signals at 7-8 ppm. What structure is likely?
Benzene ring
What IR peak distinguishes a carboxylic acid from a ketone?
The broad O-H stretch around 2500-3000 cm⁻¹
Protons appear at 7-8 ppm. What structure causes this downfield shift?
Aromatic ring (benzene)
What splitting pattern do you see for CH₂ next to CH₃?
Quartet (from the 3 H's on the methyl)
What does a UN# of 4 typically indicate?
A benzene ring (4 degrees from 3 double bonds + 1 ring)
IR shows strong peak at 1676 cm⁻¹, NMR shows signals at 7-8 ppm. What two functional groups are present?
C=O and benzene ring
An IR spectrum shows a sharp peak at 2200 cm⁻¹. What functional group is this?
C≡N (nitrile) or C≡C (alkyne)
Why would CH₂ protons appear at 3-4 ppm instead of 1.3 ppm?
They're next to an electronegative atom (O, N, or halogen - X)
If you see a doublet of doublets in the 7-8 ppm range, what structure is likely present?
Benzene ring (aromatic protons coupled to neighbors)
If UN# = 2 for a molecule, list two possible structures
2 double bonds, 2 rings, 1 triple bond, or 1 double bond + 1 ring
You see a sextet integrating for 1H. How many neighboring protons does this carbon have?
5 neighboring protons
What characteristic bond causes an absorbance band at 1670 - 1750 cm-1
C=O (carbonyl)
A signal appears at 10-12 ppm. What extremely deshielded proton is this?
R-COOH (carboxylic acid proton)
Why might you see only a singlet for protons that should show splitting?
They're equivalent/symmetrical OR next to O/N which undergoes rapid exchange
Calculate UN# for C₁₂H₁₇O₂N
UN# = (2(12)+2+1-17)/2 = 5
A molecule shows high symmetry with only 3 types of protons despite having 12 H's total. What does this suggest?
The molecule has a symmetrical structure (possibly two identical groups)